The goal of this work is to study the molecular steps essential for myeloid lineage development in order to better understand both normal hematopoiesis and leukemogenesis. The zebrafish vertebrate model was chosen since its optical clarity allows observation throughout development; genetic manipulations of the embryos can be performed easily; and phenotype-driven, forward genetic analyses are feasible. The long-term objective of this study is the analysis of myeloid development in the zebrafish and the identification and characterization of zebrafish mutants with defects in the myeloid pathway. Defined translocations have been identified in 45 percent of acute leukemias. Many of these translocations involve genes vital to normal hematopoiesis. However, the alterations responsible for the majority of leukemias are still not known. Therefore, we hypothesize that characterization of myelopoiesis and the identification of genes required for determination of the myeloid program will provide insights into the lesions in acute myeloid leukemia (AML). The specific aims of the project are as follows: 1 . To assess the function of a novel, myeloid-specific zebrafish transcription factor, c/ebp1, and to place it within the myeloid developmental pathway. Both over-expression and loss of expression approaches will be used to test the hypothesis that c/ebp1 plays an essential role in myeloid development. Currently available myeloid markers will be used for RNA in situ hybridization experiments combined with functional assays. 2. To identify zebrafish mutants with defects in myeloid development in an ENU mutagenesis. The screening will utilize whole-mount RNA in situ hybridization with the general myeloid marker, l-plastin. A zebrafish mutant lacking l-plastin expression has already been identified in our pilot screen and will be characterized and the gene identified through traditional positional cloning techniques.